The present invention relates to a light-emitting device using electroluminescence (EL).
Semiconductor lasers have been used as a light source for optical communications systems. Semiconductor lasers excel in wavelength selectivity and can emit light with a single mode. However, it is difficult to fabricate semiconductor lasers because many stages of crystal growth are required. Moreover, types of light-emitting materials used for semiconductor lasers are limited. Therefore, semiconductor lasers cannot emit light with various wavelengths.
Conventional EL light-emitting devices which emit light with a broad spectral width have been used in some applications such as for displays. However, EL light-emitting devices are unsuitable for applications related to optical communications and the like, in which light with a narrow spectral width is required.
An objective of the present invention is to provide a light-emitting device which can emit light with a remarkably narrow spectral width in comparison with conventional EL light-emitting devices and exhibiting directivity, and can be applied not only to displays but also optical communications and the like.
A first light-emitting device according to the present invention comprises a substrate and a light-emitting device section,
wherein the light-emitting device section includes:
a light-emitting layer capable of emitting light by electroluminescence;
a pair of electrode layers for applying an electric field to the light-emitting layer;
a light-transmitting section for transmitting light emitted from the light-emitting layer;
an insulation layer disposed between the electrode layers, having an opening formed in a part of the insulation layer, and functioning as a current concentrating layer for specifying a region through which current supplied to the light-emitting layer flows through a layer in the opening; and
a grating for light transmitting through the light-transmitting section.
According to this light-emitting device, electrons and holes are injected into the light-emitting layer respectively from the pair of electrode layers (cathode and anode). Light is emitted when the molecules return to the ground state from the excited state by the recombination of the electrons and holes in the light-emitting layer. The light emitted from the light-emitting layer has wavelength selectivity and directivity by the grating for light which is transmitted through the light-transmitting section, specifically, a grating in which two types of mediums having different refractive indices are arranged alternately and periodically.
The light-transmitting section is part of the light-emitting device section and supplies light obtained in the light-emitting layer of the light-emitting device section toward the waveguide section. The light-transmitting section has at least a grating section having a function of providing wavelength selectivity and a member (for example, one of the electrode layers) for connecting a core layer of the waveguide section with the grating.
According to this light-emitting device, since the insulation layer functions as a current concentrating layer in the light-emitting device section, the region where current is supplied to the light-emitting layer can be specified. Therefore, current intensity and current distribution can be controlled in the region from which it is desired to emit light, whereby light can be emitted with high emission efficiency. In the case where the insulation layer functions as cladding and the waveguide has a light-emitting layer as a core and an insulation layer as cladding, the waveguide mode of light transmitted to the waveguide section through the light-transmitting section can be controlled by specifying the opening of the insulation layer. Specifically, the waveguide mode of light transmitted through the light-emitting layer (core) can be set at a predetermined value by specifying the width of the region where light is confined (width of the opening perpendicular to the direction of light) using the insulation layer (cladding). The relation between the waveguide mode and the waveguide is generally represented by the following equation.
Nmax+1xe2x89xa7K0xc2x7axc2x7(n12xe2x88x92n22)xc2xd/(xcfx80/2)
where
K0:2xcfx80/xcex
a: half width of core of waveguide
n1: refractive index of core of waveguide
n2: refractive index of cladding of waveguide
Nmax: maximum value of possible waveguide mode
Therefore, when the parameters of the above equation such as the refractive indices of the core and cladding have been specified, the width of the light-emitting layer (core) specified by the width of the opening of the current concentrating layer may be selected according to the desired waveguide mode. Specifically, the width (2a) of the light-emitting layer corresponding to the core at a desired waveguide mode can be calculated from the above equation by substituting the refractive indices of the light-emitting layer provided inside the current concentrating layer and the insulation layer (current concentrating layer) for the refractive indices of the core and cladding of the waveguide, respectively. The width of the core layer of the waveguide section to which light is supplied from the light-emitting device section is preferably calculated taking into consideration the resulting width of the light-emitting layer, calculated value obtained from the above equation based on the desired waveguide mode, and the like. Light with a desired mode can be transmitted from the light-emitting device section to the waveguide section with high combination efficiency by appropriately specifying the width of the light-emitting layer, width of the core layer, and the like. In addition, in the light-emitting device section, light-emitting layer in the current concentrating layer formed of the insulation layer may not uniformly emit light. Therefore, it is preferable to suitably adjust the designed values of each member such as the light-emitting layer, light-transmitting section, and waveguide section based on the width (2a) of the core (light-emitting layer) calculated from the above equation so that each member exhibits high combination efficiency.
The waveguide mode of the light-emitting device is preferably 0 to 1000. In particular, when used for communications, the waveguide mode is preferably about 0 to 10. Light with a predetermined waveguide mode can be efficiently obtained by specifying the waveguide mode of light in the light-emitting layer.
A second light-emitting device according to the present invention comprises a light-emitting device section and a waveguide section which transmits light emitted from the light-emitting device section, the light-emitting device section and the waveguide section being integrally formed on a substrate,
wherein the light-emitting device section includes:
a light-emitting layer capable of emitting light by electroluminescence;
a pair of electrode layers for applying an electric field to the light-emitting layer;
a light-transmitting section for transmitting light emitted from the light-emitting layer;
an insulation layer which is disposed to be in contact with the light-transmitting section and is capable of functioning as a cladding layer; and
a grating for light transmitting through the light-transmitting section, and
wherein the waveguide section includes:
a core layer integrally formed with at least part of the light-transmitting section; and
a cladding layer integrally formed with the insulation layer.
According to the second light-emitting device, light with superior wavelength selectivity and directivity can be emitted by the same principle as that of the first light-emitting device.
In the second light-emitting device, at least part of the light-transmitting section of the light-emitting device section and the core layer of the waveguide section are integrally formed. The insulation layer (cladding layer) of the light-emitting device section and the cladding layer of the waveguide section are integrally formed. Therefore, the light-emitting device section and the waveguide section are optically connected with high combination efficiency, whereby light is efficiently transmitted.
In the case of this configuration, as a material for the insulation layer, materials which function as a cladding layer for the light-transmitting section are selected. According to the light-emitting device having this configuration, the light-transmitting section of the light-emitting device section and the core layer of the waveguide section can be formed and patterned in the same step, thereby simplifying the fabrication. The insulation layer (cladding layer) of the light-emitting device section and the cladding layer of the waveguide section can be formed and patterned in the same step, thereby also simplifying the fabrication.
In the first and the second light-emitting devices, the opening of the insulation layer is preferably a slit extending in the periodic direction of the grating, specifically, in the direction to which light is waveguided. At least part of the light-emitting layer is preferably formed in the opening formed in the insulation layer. According to this configuration, the region of the light-emitting layer to which it is desired to supply current and the region specified by the current concentrating layer can be self-alignably positioned.
In the first and second light-emitting devices, the grating is preferably a distributed feedback type grating or a distributed-Bragg-reflection-type grating. Light emitted from the light-emitting layer is caused to resonate by forming such a distributed feedback type grating or distributed-Bragg-reflection-type grating, whereby light having wavelength selectivity, narrow emission spectral width, and excellent directivity can be obtained. In these gratings, the pitch and depth of the grating are set depending on the wavelength of light to be emitted.
Moreover, emission of light with a single mode can be ensured by providing a distributed feedback type grating with a xcex/4 phase shift structure or a gain-coupled structure. xe2x80x9cxcexxe2x80x9d used herein represents the wavelength of light in the light-transmitting section.
A distributed feedback type grating having a xcex/4 phase shift structure or a gain-coupled structure is a preferable configuration common to the light-emitting devices according to the present invention. It is sufficient for the grating to achieve the above functions, and the region for forming the grating is not limited. For example, the grating may be formed in either the light-transmitting section or in a layer in contact with the light-transmitting section.
The light-emitting layer preferably includes an organic light-emitting material as a light-emitting material. Use of organic light-emitting materials widens the selection range of the material in comparison with the case of using semiconductor materials or inorganic materials, for example. This enables emission of light with various wavelengths.
The light-emitting device according to the present invention may have various structures. Examples of typical structures will be given below.
(a) In a light-emitting device according to a first structure, the light-emitting device section may comprise:
a transparent anode which is formed on the substrate and is capable of functioning as at least part of the light-transmitting section,
a grating formed in part of the anode;
an insulation layer having an opening facing the grating,
a light-emitting layer, at least part of the light-emitting layer being formed in the opening of the insulation layer; and
a cathode.
(b) In a light-emitting device according to a second structure, the light-emitting device section may comprise:
a grating formed in part of the substrate;
a transparent anode which is formed on the grating and is capable of functioning as at least part of the light-transmitting section;
an insulation layer having an opening facing the anode;
a light-emitting layer, at least part of the light-emitting layer being formed in the opening of the insulation layer; and
a cathode.
(c) In a light-emitting device according to a third structure, the light-emitting device section may comprise:
a grating substrate disposed on the substrate, a grating being formed in part of the grating substrate;
a transparent anode which is formed on the grating of the grating substrate and is capable of functioning as at least part of the light-transmitting section;
an insulation layer having an opening facing the anode;
a light-emitting layer, at least part of the light-emitting layer being formed in the opening of the insulation layer; and
a cathode.
The light-emitting devices according to the first to third structures preferably further have a waveguide section integrally formed with the light-emitting device section. The waveguide section has a core layer formed on the substrate or the grating substrate and includes a core layer optically continuous with the anode, and a cladding layer which covers the exposed area of the core layer and is optically continuous with the insulation layer.
As described above, according to the present invention, a light-emitting device which can emit light having a wavelength with a remarkably narrow spectral width in comparison with conventional EL light-emitting devices and exhibiting directivity, and can be applied not only to displays but also optical communications and the like can be provided.
Some of the materials which can be used for each section of the light-emitting device according to the present invention will be illustrated below. These materials are only some of the conventional materials. Materials other than these materials can also be used.
(Light-emitting Layer)
Materials for the light-emitting layer are selected from conventional compounds to obtain light with a predetermined wavelength. As the materials for the light-emitting layer, many organic and inorganic compounds may be used. Of these, organic compounds are preferable in view of availability of wide variety of compounds and film-formability. Various materials can be selected by using organic light-emitting materials in comparison with the case of using semiconductor materials or inorganic materials, for example. This enables light with various wavelengths to be emitted.
As examples of such organic compounds, aromatic diamine derivatives (TPD), oxydiazole derivatives (PBD), oxydiazole dimers (OXD-8), distyrylarylene derivatives (DSA), beryllium-benzoquinolinol complex (Bebq), triphenylamine derivatives (MTDATA), rubrene, quinacridone, triazole derivatives, polyphenylene, polyalkylfluorene, polyalkylthiophene, azomethine zinc complex, polyphyrin zinc complex, benzooxazole zinc complex, and phenanthroline europium complex which are disclosed in Japanese Patent Application Laid-open No. 10-153967, and the like can be given.
Moreover, as materials for the organic light-emitting layer, conventional compounds disclosed in Japanese Patent Application Laid-open No. 63-70257, No. 63-175860, No. 2-135361, No. 2-135359, No. 3-152184, No. 8-248276, No. 10-153967, and the like can be used. These compounds can be used either individually or in combination of two or more.
As examples of inorganic compound, ZnS:Mn (red region), ZnS:TbOF (green region), SrS:Cu, SrS:Ag, SrS:Ce (blue region), and the like can be given.
(Optical Waveguide)
The optical waveguide has a layer which functions as a core, and a layer which has a refractive index lower than that of the core and functions as cladding. Specifically, these layers include the light-transmitting section (core) and insulation layer (cladding) of the light-emitting device section, core layer and cladding layer of the waveguide section, substrate (cladding), and the like. Conventional inorganic and organic materials can be used for the layers for forming the optical waveguide.
As typical examples of inorganic materials, TiO2, TiO2xe2x80x94SiO2 mixture, ZnO, Nb2O5, Si3N4, Ta2O5, HfO2, ZrO2, which are disclosed in Japanese Patent Application Laid-open No. 5-273427, and the like can be given.
As typical examples of organic materials, various conventional resins such as thermoplastic resins, thermosetting resins, and photocurable resins can be given. These resins are appropriately selected depending on the method of forming the layer and the like. For example, use of a resin cured by energy from at least one of heat or light enables utilization of commonly used exposure devices, baking ovens, hot plates, and the like.
As examples of such materials, a UV-curable resin disclosed in Japanese Patent Application No. 10-279439 applied by the applicant of the present invention can be given. As UV-curable resins, acrylic resins are preferable. UV-curable acrylic resins having excellent transparency and capable of curing in a short period of time can be obtained by using commercially-available resins and photosensitizers.
As specific examples of basic components of such UV-curable acrylic resins, prepolymers, oligomers, and monomers can be given.
Examples of prepolymers or oligomers include acrylates such as epoxy acrylates, urethane acrylates, polyester acrylates, polyether acrylates, and spiroacetal-type acrylates, methacrylates such as epoxy methacrylates, urethane methacrylates, polyester methacrylates, and polyether methacrylates, and the like.
Examples of monomers include monofunctional monomers such as 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, N-vinyl-2-pyrrolidone, carbitol acrylate, tetrahydrofurfuryl acrylate, isobornyl acrylate, dicyclopentenyl acrylate, and 1,3-butanediol acrylate, bifunctional monomers such as 1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate, neopentyl glycol diacrylate, neopentyl glycol dimethacrylate, ethylene glycol diacrylate, polyethylene glycol diacrylate, and pentaerythritol diacrylate, and polyfunctional monomers such as trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, pentaerythritol triacrylate, and dipentaerythritol hexaacrylate.
These inorganic and organic materials are illustrated taking only light confinement into consideration. In the case where the light-emitting device section has a light-emitting layer, hole transport layer, electron transport layer, and electrode layer and at least one of these layers functions as the core or cladding layer, the materials for these layers can also be employed as the material for the layers of the optical waveguide.
(Hole Transport Layer)
When using an organic light-emitting layer in the light-emitting device section, a hole transport layer may be formed between the electrode layer (anode) and the light-emitting layer, as required. As the materials for the hole transport layer, materials conventionally used as hole injection materials for photoconductive materials or materials used for a hole injection layer of organic light-emitting devices can be selectively used. As the materials for the hole transport layer, any organic and inorganic substances having a function of either hole injection or electron barrier characteristics may be used. As specific examples of such substances, substances disclosed in Japanese Patent Application Laid-open No. 8-248276 can be given.
(Electron Transport Layer)
When using an organic light-emitting layer in the light-emitting device section, an electron transport layer may be formed between the electrode layer (cathode) and the light-emitting layer, as required. Materials for the electron transport layer are only required to have a function of transporting electrons injected from the cathode to the organic light-emitting layer. Such materials can be selected from conventional substances. For example, substances disclosed in Japanese Patent Application Laid-open No. 8-248276 can be given as specific examples.
(Electrode Layer)
As the cathode, electron injectable metals, alloys, electrically conductive compounds with a small work function (for example, 4 eV or less), or mixtures thereof can be used. Materials disclosed in Japanese Patent Application Laid-open No. 8-248276 can be given as specific examples of such electrode substances.
Metals, alloys, electrically conductive compounds with a large work function (for example, 4 eV or more), or mixtures thereof can be used as the anode. In the case of using optically transparent materials as the anode, transparent conductive materials such as CuI, ITO, SnO2, and ZnO can be used. In the case where transparency is not necessary, metals such as gold can be used.
In the present invention, there are no specific limitations to the method of forming the grating and conventional methods can be employed. Typical examples of such methods will be given below.
(1) Lithographic Method
The grating is formed by irradiating a positive or negative resist with ultraviolet rays, X-rays, or the like and developing the resist thereby patterning the resist layer. As a patterning technology using a resist formed of polymethylmethacrylate or a novolak resin, technologies disclosed in Japanese Patent Applications Laid-open No. 6-224115 and No. 7-20637 can be given.
As a technology of patterning a polyimide using photolithography, technologies disclosed in Japanese Patent Applications Laid-open No. 7-181689 and No. 1-221741, and the like can be given. Furthermore, Japanese Patent Application Laid-open No. 10-59743 discloses a technology of forming a grating from polymethylmethacrylate or titanium oxide on a glass substrate utilizing laser ablation.
(2) Formation of Refractive Index Distribution by Irradiation
The grating is formed by irradiating the optical waveguide section of the optical waveguide with light having a wavelength which causes changes in the refractive index, thereby periodically forming areas having different refractive indices in the optical waveguide section. As such a method, it is preferable to form the grating by forming a layer of polymers or polymer precursors and polymerizing part of the polymer layer by irradiation or the like, thereby periodically forming areas having different refractive indices. Such a technology is disclosed in Japanese Patent Applications Laid-open No. 9-311238, No. 9-178901, No. 8-15506, No. 5-297202, No. 5-32523, No. 5-39480, No. 9-211728, No. 10-26702, No. 10-8300, and No. 2-51101, and the like.
(3) Stamping Method
The grating is formed by hot stamping using a thermoplastic resin (Japanese Patent Application Laid-open No. 6-201907), stamping using an UV curable resin (Japanese Patent Application Laid-open No. 10-279439), stamping using an electron-beam curable resin (Japanese Patent Application Laid-open No. 7-235075), or the like.
(4) Etching Method
The grating is formed by selectively removing a thin film using lithography and etching technology, thereby patterning the film.
Methods of forming the grating are described above. In summary, the grating only has two areas with different refractive indices. The grating may be formed using a method of forming these two areas from two materials having different refractive indices, a method of forming these two areas having different refractive indices from one material by partially modifying the material, and the like.
Each layer of the light-emitting device may be formed using a conventional method. For example, each layer of the light-emitting device is formed using a suitable film-forming method depending on the materials therefor. As specific examples of such a method, a vapor deposition method, spin coating method, LB method, ink jet method, and the like can be given.